ERH is an in situ technology that heats subsurface soil and groundwater, usually for the purpose of removing environmental contaminants such as volatile organic compounds (VOCs). Common VOCs include many halogenated compounds—industrial solvents such as trichloroethene (TCE) and pesticides such as ethylene dibromide. As the subsurface is heated, VOCs are driven into the vapor phase and soil moisture is boiled into steam. The resulting steam and VOC vapor mixture is removed from the subsurface by applying a vacuum that draws the gases into a well and pulls them to the surface for treatment.
ERH uses the heat generated by the resistance of the soil matrix to the flow of electrical current to raise subsurface temperatures. ERH is equally effective in water saturated and unsaturated (vadose zone) soils. To implement the technology, electrodes are placed into the ground so that they are spaced throughout a targeted contaminated region. The vertical limits for ERH are set by the depth to which boreholes for electrode construction can be drilled. Alternatively, electrodes can be installed by pile driving—inserting a circular pipe or sheet pile into the subsurface.
The ERH electrodes conduct electrical current into the subsurface and are designed to input electrical current into the targeted depth interval. The subsurface interval that is exposed to electrical resistance heating is called the conductive interval. In the conductive interval, the electrode construction materials are non-insulated and the borehole annulus is packed with a conductive material to increase the effective diameter of the electrode. In those portions of the subsurface where electrical resistance heating is not required, the electrode construction materials are electrically insulated and the borehole annulus is filled with relatively non-conductive materials such as sand, bentonite, or neat cement grout. Drilled electrodes are typically 8-12 inches in diameter and spaced 12-24 feet apart, though sizes and spacing outside of these common ranges are occasionally used.
FIG. 1 depicts an illustrative simple bored (drilled) electrode, referred to as a “pipe electrode”. A borehole 100 is created. A galvanized or low carbon steel screen 104 having a diameter of 4 inches is inserted in borehole 100 and extends from the total depth. A galvanized or low carbon steel casing 102 also having a diameter of 4 inches, is inserted into borehole 100, beginning at the top of screen 104 and extending to just above grade surface. In the provided example, screen 104 is surrounded with conductive backfill 106, however, the conductive backfill depth interval does not usually coincide with the length of the screened interval as shown here. The conductive backfill interval 106 is the region where the electrical current flows through the soil. A layer of Bentonite clay 108 or other relatively non-conductive material, surrounds galvanized casing 102 just above galvanized screen 104. Above Bentonite clay 108 is a layer of cement grout 110. An oversleeve 112 having a diameter of approximately 8 inches and preferably made of chlorinated polyvinyl chloride (CPVC) surrounds the upper portion of galvanized casing 102. Within the depth of oversleeve 112 are the following layers stacked upwardly: sand 114, conductive backfill 116, bentonite clay 118 and cement grout 120. Sand layer 114 is typically fine masonry sand and is inserted in the borehole to prevent the upper particle layers from sinking into the wet cement grout as they are added to the borehole during construction. The upper section of conductive backfill 116 is the subsurface neutral. The subsurface neutrals for all electrodes are shorted together by a small wire (not shown)—this damps down the near-surface voltage gradients for personnel safety. Bentonite clay layer 118 prevents liquid cement grout 120 from seeping down into permeable conductive layer 116 as the liquid grout is added to the borehole during construction; alternatively, fine masonry sand could be used instead of Bentonite clay. A power cable 122 is connected to electrode 100 to conduct an electrical current. It will be understood by those skilled in the art that other electrode configurations are possible, including different types of conductive and non-conductive materials.
In the provided example electrode, a thermocouple 124 is situated to monitor the temperature of gases that are being extracted from electrode 100 by vacuum. It is often also necessary to monitor temperatures at various depths in the subsurface. Such temperature monitoring can be performed using temperature monitoring points (TMPs) that contain a plurality of thermocouples.
FIG. 2 depicts an illustrative plot plan of a remediation site with a treatment area 210. Circles 202 represent electrodes, which also serve as vapor recovery wells. Triangular marking 204 represents a TMP. Diamond 206 is a vapor recovery well that does not serve as an electrode. Shaded circles 208 are co-located monitoring wells and TMPs.
An ERH power control unit (PCU) is used to control the voltage that is applied to the subsurface. Each electrode differs in electrical phase from all of the electrodes that surround it and will thus conduct current to adjacent out-of-phase electrodes. Resistance by the subsurface environment to this flow of electrical current heats the soil and groundwater between the electrodes.
However, the electrical current does not uniformly heat the subsurface. Geometry causes the current flux or current density to be higher near the electrodes. This effect is most apparent when considered from a plan view perspective as shown in FIG. 3. The regions within a few inches of electrodes 302 have a higher current flux as shown by the broken lines, and therefore heat more quickly than other regions. This non-uniform heating is undesirable because it is less energy efficient. In addition, most applications of ERH require heating to the boiling temperature of water. A higher current flux at the electrode-soil interface would lead to stronger boiling there. If the soil immediately adjacent to the electrode begins to dry out, then its local resistance will begin to increase; this can lead to a vicious cycle—more resistance, more heating, more drying, more resistance, etc. Electrode dryout can lead to a condition in which the electrode becomes essentially non-conducting.
It is common practice to drip water into the vadose zone portion of electrodes in order to combat electrode dryout. Alternatively, the electrode may be cooled to prevent dry out. It is also common practice to add conductive ions to the electrode, either as solids or liquids during installation of the electrode or in water-dissolved form in conjunction with electrode drip water. The conductive ions reduce the local soil resistance, reduce local heating, and ease dry-out problems. One could say that drip water and electrode cooling treat the symptom (higher heat due to greater current flux) while adding conductive ions treats the problem by reducing the heat generated by the higher local current flux. In theory, though never achievable in practice, one could establish the proper conductivity gradient with radial distance from the electrode to match the current flux over that radial distance to provide absolutely uniform heat generation. In this ideal scenario, the concentration of conductive ions should be inversely proportional to radial distance (for a cylindrical electrode)—i.e. the slope of the gradient should be steep.
The addition of conductive fluids has two disadvantages:                1. If conductive ions are added only during electrode installation, then diffusion of the ions will reduce their concentration over time and reduce the impact of the addition. Even if the ions are added in conjunction with drip water, the flow of the drip water itself can easily transport the ions away from the electrode. Under either scenario, the slope of the concentration gradient tends to be too broad and flat.        2. All conductive ions (at least at effective concentrations) are considered to degrade the quality of the groundwater and are to some extent counter-productive to the goal of restoring the groundwater to optimal conditions.        
Accordingly, there is a need to produce a conductive ion concentration gradient that more closely approaches the theoretical ideal to enhance ERH and related remediation processes.